The most important thing to do in order to prevent Melasma is to use a high degree of Sunscreen. Most cases could have been avoided with proper instruction of the photoprotection needed when pregnant. If you develop melasma, either from pregnancy or birth control pills there are a few very g...
Long term use of Botox injections will not permanently weaken or paralyze the muscles. The muscle activity will resume and with time, the effect will be as it was prior to the first treatment with Botox.
There is no problem with you getting Botox post Splenectomy and while on Erythromycin.
I would be quite concerned about more invasive procedures such as Laser Resurfacing or a Facelift, since the absence of your spleen would make you very susceptible to infections....